Physiological performance of two forms of lacustrine brook charr, Salvelinus fontinalis, in the open-water habitat

Although morphological differences among ecotypes have often been observed, little is known about the adaptive value of these differences in terms of the relative performance of individuals. The first objective of this study was to compare specific growth rates, muscle proteins, lipids, and water content in the muscles and digestive tracts of littoral and pelagic brook charr (1+) when individuals of both forms were restricted to feeding on zooplankton for 76 days, in pelagic enclosures of a Canadian Shield lake. The second objective was to determine if differences in the physiological performance of experimental fish were related to differences in their morphology. The specific growth rate was low and did not differ significantly between littoral and pelagic individuals at the end of the experiment. However, littoral individuals lost more lipids and had lower muscle protein content than pelagic fish at the end of the experiment. The pectoral fin was longer in littoral individuals than in pelagic ones. Furthermore, the biochemical discriminant scores of individuals were marginally correlated to those of morphology. Based on a comparison of their proximate tissue composition, our results suggest that trophic diversification is adaptive in brook charr because littoral individuals exhibited lower physiological performance than pelagic ones when restricted to feeding in the pelagic zones. In addition, our results support that morphological descriptors are related to physiological performance. This study is one of the first field attempts to relate differences in trophic morphology of a fish species to physiological performance.